Presently there is a substantial effort in the US and in laboratories around the world in intense microwave source research. This paper reviews the current sate of the art and technical barriers to further progress in producing higher power and higher energy pulses of radiofrequency radiation. The paper also suggests research avenues that offer the potential for further improvements in these sources. Key recent publications and conference proceedings are cited that provide a guide to current thought and recent advances in high power microwave source research.

A variety of cesium iodide-coated carbon fiber cathodes have been designed and tested for high voltage (300 - 500 kV), high current operation. Here, we describe the conditioning process and present the most recent experimental data for a thin annular beam test diode. During these reproducible microsecond-duration pulses, this diode exhibits promising beam impedance maintainability and gap closure speeds of less than 0.4 cm/microsecond(s) ec. Also presented are recent results for a carbon fiber cathode tested in a relativistic klystron oscillator and future plans for such a cathode in the magnetically insulated line oscillator.

A theory relating phase of the current modulation to the microwave phase is developed for the folded waveguide klystrons. The power transfer from the beam to microwave is described in terms of the propagation distance and phase shift (alpha) _n of the energy modulations launched at gaps. It is shown that the phase difference between the current modulation and the induced voltage at the gaps is essential for efficient power transfer from the beam's kinetic energy to microwave field energy. The optimum value of the phase shift for maximum power transfer from the beam to microwaves occurs at (alpha) _n equals -2, which may be useful for design of the folded waveguide klystron.

An important issue in the design of high-power microwave sources is how to prevent high-intensity relativistic electron beams from forming halos because they cause electron beam losses and radio-frequency pulse shortening. In this paper, we study the behavior of a high-intense electron beam under a current-oscillation-induced mismatch between the beam and the magnetic focusing field, using a 2D self-consistent electrostatic model. For high-intensity electron beams, it is found from the self-consistent simulations that sizable halos appear after the beam envelope undergoes several mismatched oscillations, depending on the amplitude of mismatched beam envelope oscillations. Detailed simulation results are presented for the choice of system parameters corresponding to the 50 MW, 11.4 GHz periodic permanent magnetic focusing klystron experiment at the Stanford Linear Accelerator Center. Moreover, it is found that the process of halo formation in a uniform-solenoidal focusing klystron is essentially the same as in the PPM focusing klystron with comparable system parameters.

An ultra-wideband, nonperturbing, electric-field sensor is being developed that uses the linear electro-optic effect and is packaged in a suitcase-sized optical configuration. The methodology has been demonstrated on the optical bench with ZnTe and used to measure an applied electric field. The immediate goal is to demonstrate the sensor up to 4 GHz and apply it to unknown fields in a configuration using polished and coated ZnTe crystals. The unique features of the sensor include non-perturbing rf CW & pulsed (< 2 GHz) measurements inside of cavities, while preserving the amplitude, polarization, and phase content of the detected signal. Preliminary measurements reported here verify the crystal sensitivity of approximately 3 V/cm (1 mW/cm²) (into a 5 GHz bandwidth (BW) oscilloscope), and response linearity. Variations in optical configurations are compared on the basis of sensitivity.

The Microwave-to-Optical Transformation (MOT) method, reported previously, makes use of an optical thin film filter design took known as the Optical Admittance Diagram, the characteristic matrix and the quarter wave rule. Stripline elements are re-configured and modeled as sequential quarter wave optically thin film layers. The previous work was qualitative and mainly focused on the intuitively useful analysis that the MOT method offers the microwave engineer. A more rigorous formalism shall be presented in this paper which allows the computation of E- Field, both amplitude and phase. In addition, new graphical techniques that provide the designer further insight will accompany the E-field analysis. This is a novel extension of the previous work. Microwave and optical computing circuits and components that span a vast wavelength range may be adaptable to this technique depending on their specific utilization. This paper will first develop an extension of the MOT method for electric field strength characterization. Secondly, this technique will be applied to the Wilkinson Power Divider used in the previous work. Use of this already MOT-characterized stripline component will validate the utility of this technique by also analyzing the electric field through the same simple microwave circuit. Finally, this technique will be applied to the design of a co-planar waveguide transition component used for launching power to a Field Emitter Array.

High Power Impulse generation depends strongly on both the voltage and the rate of voltage rise which is applied to the radiating structure. Thus, placing the peaking switch at the apex of the antenna optimizes the transient radiating system. Liquids are prime candidates for the peaking switch media because of their high dielectric strength and its compatibility with lensing structures.

Physical Wavelets are exact, caused pulsed-beam solutions of the inhomogeneous wave- or Maxwell equations whose `wavelet parameters' specify the point of emission, launch time, radius of the emitting aperture, direction of propagation, duration and width of the pulse. We describe their time- domain radiation patterns, which show that the beams can be made arbitrarily well focused by choosing the wavelet parameters accordingly. We also find the source distribution as a generalized function supported on the disk aperture determined by the wavelet parameters. As the radius of the disk shrinks to zero, the distribution reduces to the usual point source represented by the retarded Green function. It is suggested that such `physical wavelets' may be synthesized in practice by realizing their source distributions.

I calculate the recoil force on the linear dynamic polarization current of a relativistic test particle participating in collective bremsstrahlung in a nonequilibrium beam-plasma system. Possible contributions arise both from terms involving direct interaction in second order of the field of the test particle with its own induced linear polarization, and also from interaction of the field scattered by the second-order dynamic polarization with the linear polarization.

A stability analysis is made of an electron beam, propagating along and gyrating about a uniform magnetic field, for the case of a spatiotemporal equilibrium distribution in the phase angle of the transverse electron momentum component. The axial momentum component and the magnitude of the transverse momentum component are assumed to have definite values in the equilibrium distribution. The analysis is carried out by applying Lorentz transformations to previous results for nongyrotropic equilibrium distributions. The dielectric matrix, its eigenmodes (which relate field amplitudes), and the dispersion relation are obtained. Numerical results indicate that introducing a spatiotemporal equilibrium distribution has only a small effect on maximum growth rates of radiation, but has a strong effect on the frequencies and wave numbers at which instability occurs.

It is at very strong magnetic fields that the helical Cerenkov effect, originating from the electron guiding center for an electron in helical motion in a magnetic field which is superimposed on a dielectric medium, resembles most closely the ordinary Cerenkov effect. In the absence of extremely strong magnetic fields in the laboratory, we turn our attention to the neutron stars (pulsar) and supernovas which can have magnetic fields whose values can easily be in the range of 10⁵ - 10⁹ T. The medium in which these magnetic fields reside is likely to be an ionized medium; that is, a plasma, which, as usual, may be assumed to be dominated by electrons. Here we wish to argue that in such a strong magnetic field dominated medium, at least on a classical level, radiation process associated with the helical Cerenkov radiation could be rather important.

Waveguide structure with an inner rod supporting slow-wave propagation allows one to combine a TWT-amplifier and a surface wave radiating antenna in a single hybrid device. Such a device could significantly extend the range of possibilities for microwave beam control at the high level of radiated power. In the work presented, the linear theory is developed for a relativistic Cherenkov amplifier, which configuration is a circular waveguide with an inner dielectric rod and an annular electron beam between the rod and outer wall. Dielectric rod antennas typically operate in the fundamental non-axisymmetric HE₁₁ mode, therefore, the dispersion relation is derived for the general case of non-axisymmetric perturbations. The achievable gain and bandwidth are determined from the numerical solutions of the dispersion relation for various waveguide and beam parameters. Spatial growth rates for the HE₁₁ mode, symmetric TM₀₁ mode (usual TWT operating mode), and other high-order modes are compared. Investigations of the parameter space allow one to find sets of parameters where the HE₁₁ mode dominates. The gain and bandwidth values in this case can be typical for usual relativistic TWTs at accessible beam currents and voltages and reasonable geometries for the X-band frequencies and above. Key characteristics of the proposed concept for a high-power microwave source are formulated, and possible advantages of a hybrid antenna-amplifier are discussed.

In this paper, the retard effect, lifetime and coherent condition of the superradiance pulses emitted by prebunched electron beams are discussed.

The space-charge limiting current of coaxial drift tube is derived by the Green's function method. 2D feature of coaxial vircator is analyzed within the framework of steady- state relativistic-fluid maxwell equations. The radial current arriving at the anode, and plasma frequency for electron beam can be obtained. The resulting plasma frequency solution agrees with the PIC simulational frequency, MAGIC, a 21/2 dimensional particle-in-cell code are used to simulate the coaxial vircator. The output power, the FFT of output power and the plot of single frequencies spectrum, and the time history of current are presented. GW output power is generated with 0.6 MV voltage, and microwave radiation efficiency is about 14.40%.

Interaction of relativistic electron beam with the electromagnetic field of two longitudinal modes near the (pi) -type oscillations of TM₀₁-mode has been investigated by means of a nonlinear code in the overmoded one- and two-sectional slow-wave structures (SWS). Longitudinal mode interaction in Cerenkov one-sectional generators has shown to result in the changes of generation regime from the stable to unstable one. For a multiwave Cerenkov generator with a two-sectional SWS a generation regime classification depending on the type of longitudinal mode interaction has been carried out. Numerical simulation results are compared with the previously obtained experimental data.

Some of the effects in a weak focusing magnetic field are considered and the degrees of their influences are estimated in the report. The effects are divided into three groups. The first one is connected with fast cyclotron waves, the second--with the process of electron beam formation and the third--with the collecting of electrons. It is shown that the operating backward electromagnetic wave and the extracting forward electromagnetic wave, which extracts the microwave energy from the interaction space, have the difference in the process of reradiation into cyclotron waves. The report includes results of calculation of the equations together with the deduction of those equations. The influence of decompression on the BWO operation driven by weakly magnetized electron beam is explained. The results of experiments with the specially elaborated model are presented. The last problem is connected with virtual cathode that can appear in the collector region and reflect back the electrons. The influence of the reflected electrons is estimated.

The purpose of this work is to theoretically understand what are the possible noise levels in a magnetron or a crossed- field amplifier, due to parametric three-wave interactions in the electron plasma, at various operating parameters. Our approach is to use the cold-fluid equations and their Fourier decomposition, into a background mode, a pump (RF) mode, and two other noise (RF) modes. The two RF noise modes are assumed to interact parametrically with the large RF pump mode, and to satisfy the standard resonance conditions for the sum of the wave vectors and sum of the frequencies.

During the recent years, the application of chaotic systems and their signals have become a subject of intensive research. Researchers have made a great progress in the application of chaos in secure communication, associative memory, and optimum computation etc. In this paper we will use the `noise-like' property of chaos to generate broadband signals, that means we will treat chaos as the broadband sources. As we all know, we cannot distinguish chaos and noise only by their observable time series. The series of chaotic signals seem to be random and their spectrum is broad. So it inspires us to generate broadband signals through chaotic dynamics. The methods are various and they can be realized by hardware easily. The important thing must to be stressed is that we must decide which kind of method to be used due to the corresponding application. For example, in noise radar we need the noise signal to be modulated and transmitted, so the chaotic signal from the broadband source must have the best correlation property and distribution property etc. Use chaos to generate broadband signals is convenient, it is a new develop orientation in such field.

On the basis of the coupled wave equation the transformation of symmetrical electrical modes TM₀₁ and TM₀₂ mixture into the mixture of TE₁₁- and TE₁₂-modes having a polarization close to a linear one has been investigated. Mode transformation was realized by a converter consisting of the pieces of a curved overmoded circular waveguide with an alternating curvature.

Numerical investigations of electrodynamical systems (EDS) for experimental relativistic Backward Wave Oscillators are made with help of computer codes based on linear theory methods. Dispersion curves for the nonaxisymmetric eigenwaves of the corrugated slow-wave structures (SWS) are calculated. Resonance properties of an experimental EDS containing finite length corrugated SWS, diaphragm cutoff neck and conical horn antenna, are investigated in detail. Obtained results are discussed and compared with known theoretical and experimental data.